84 



■becomes possible by means of a peculiar development of the antiambulacral side of the 

 disc which hereby, quite contrarily to what is the case in the other course of development, 

 acquires a preponderance over the ambulacral side ; a phenomenon especially manifested in 

 the Crinoids, in which the so-called calix, with the articulated stem proceeding from the 

 same, is attributable to such a development. In connexion with this centralisation of the 

 digestive system, there stands also a centralisation of another organic system, namely the 

 blood-system; as the cavities of the arms become at the same time narrower and narrower; 

 so that the perivisceral blood-cavity is likewise, for the greater part, limited to the disc. 

 We may already see evident signs of the centralisation referred to in real star-lishes, namely 

 in the remarkable fossil form Protaster; but we do not see the full development of it until we 

 come to the Ophiurans and Crinoids, in which the arms, by the above-mentioned reduction 

 of 2 of the most important organic systems, have lost so much of their original indepen- 

 dence, that they have become mere appendices of the disc, or simple organs. The greatest 

 centralisation of the cormus attained in this manner, is probably exhibited in the Ophiurans 

 and in their relatives the P]uryal8e, in which one more important organic system has aban- 

 doned its original place in the arras and centralised itself in the cavity of the disc, namely 

 the generative organs. But in the Crinoids these organs are still quite separate from the 

 disc; for which reason we must also consider these Echinoderms as less completely centra- 

 lised than the Ophiurans, and therefore of more ancient origin, which is also in some degree 

 indicated by the rather variable number of arms in the sea-lilies; and we are likewise led by 

 Paleontological deductions to the same conclusion; as evident Ophiurans do not occur until 

 long after the Crinoids; although the course of development which seems to tend towards 

 the ophiurean type is found manifested at a very early period, namely in the ancient star- 

 tish Protaster. The articulated stem peculiar to the Crinoids must be assumed to be a 

 characteristic acquired at an early period in consequence of special conditions of existance, 

 and continued in the younger stage even in the Antedon, which is free when fully deve- 

 loped. The genus Antedon must therefore, in spite of its habitual resemblance to the 

 Euryalae among the Ophiuridae, be considered as a further developed divergent branch of 

 the genuine Crinoids. 



It is thus evident that in the first course of development, which we are able to trace 

 through the groups of star-fishes, sea-urchins and sea-cucumbers, the centralisation of the 

 body is chiefly manifested in a reduction of the arms brought about by concrescence. In the 

 other course of development we have on the contrary an instance of even further develop- 

 ment of the arms, without however any decentralisation of the cormus taking place. This 

 further development of the arms may consist partly in an increased reproduction of joints 

 or metamera, whereby the arms may often attain a really extraordinary length in proportion 

 to the diameter of the disc (for instance in the genera Amphiura and Ophiopeltis belonging 

 to the Ophiuridse) and it may consist partly in a ramification of the arms, which may either 

 be a more or less extensive dichotomic splitting, or a development of alternating lateral 



